Print heads employed in inkjet printers and the like usually each contain a plurality of discharge elements arranged in (a) linear array(s) parallel to the propagation direction of the image-receiving member (typically paper) or in other words the sub scanning direction. The discharge elements usually are placed substantially equidistant from each other. In operation, the discharge elements are controlled to the image-wise discharge of ink droplets on an image-receiving member so as to form columns of image dots of ink in relation to the linear arrays. The discharge activation may be thermally or thermally assisted and/or mechanically or mechanically assisted and/or electrically or electrically assisted, including piezoelectrically. In scanning inkjet printers, the print heads are supported by a print carriage which is movable across the image-receiving member, i.e. in the direction perpendicular to the propagation direction of the image-receiving member or in other words the main scanning direction. In operation a scanning inkjet printer forms a matrix of image dots of ink corresponding to a part of an image by scanning the print heads at least once, optionally bi-directionally, over the image-receiving member in the main scanning direction. After a first matrix is completed the image-receiving member is displaced to enable the forming of the next matrix. This process may be repeated till the complete image is rendered.
When multiple print heads are employed, due to small deviations between the print heads, including e.g. dimensional variations, variations in the control of the print heads, and variations in the visco-elastic properties of the ink, the size of the image dots resulting from distinct print heads may vary on the image-receiving member. Examples of dimensional variations include differences in nozzle shape or size and differences in the shape or size of the ducts connecting the ink reservoirs with the respective nozzles. These differences may be introduced by the manufacturing process or may arise during extended use e.g. caused by contamination of the ink. An example of a variation in control is e.g. a small deviation in amplitude, shape or timing of the stimulus initiating the discharge of a discharge element. Any variation in the output parameter of distinct print heads, such as e.g., the ink dot size, or the optical density of the image formed, or dot positioning, may cause visual disturbances in the image which is formed. These disturbances are particularly annoying when the distinct print heads discharge ink of the same color. Such variation may be attributed to the print head temperature. In addition to the small deviations between the print heads, as described above, causing static variations, dynamic variations between distinct print heads may also arise, e.g. because of differences in coverage of the image parts which are to be reproduced by the distinct print heads.
In U.S. Pat. No. 6,283,650 a method is disclosed for controlling output levels of an inkjet printer having multiple print heads. Specifically, a dynamic print head temperature control method is disclosed wherein a predetermined relationship between output levels of multiple print heads is maintained by controlling the relative temperature differences between the print heads. To enable this, based on the obtained temperature of an arbitrary one of the multiple print heads, initial target temperatures for each of the multiple print heads are determined. When printing, these target temperatures are dynamically adjusted in order to maintain the predetermined relationship between the output level of the one of the multiple print heads and the output level of each of the multiple print heads.
A disadvantage of the approach as disclosed in U.S. Pat. No. 6,283,650 is that in order to maintain the predetermined relationship in output level, the relative temperature differences between distinct print heads should be that high that the proper functioning of individual print heads is hampered because the target temperature value of the print head is too low or too high. Particularly, when the temperature of a print head is too high a severe deterioration of the print quality may occur due to an increase in dot size and/or the failure of the individual discharge elements due to contamination, whereas when the temperature of a print head is too low, a severe deterioration of the print quality may occur due to a decrease in dot size and/or the failure of individual discharge elements due to the destabilisation of the discharge process. A further disadvantage of the approach as disclosed in U.S. Pat. No. 6,283,650 is that the control, drive and sensing means required to implement such a dynamic control are complex and costly. In operation, the temperature of the print heads rapidly and gradually increase, which affects the output level of the distinct print heads in different ways. According to the approach as disclosed in U.S. Pat. No. 6,283,650, the temperature of each print head needs to be accurately sensed and fed back to a controller which, after consulting predetermined target temperature tables, needs to adequately adjust the temperature of each of the distinct print heads to maintain a predetermined relationship in the output level. To be effective, a sufficiently fast rate temperature adjustment is required, or in other words the time interval between two subsequent adjustments should be small, and the adjustment time should be sufficiently small in order to obtain a more or less continuous temperature adjustment. This is particularly challenging when a print head needs to be cooled to obtain its target temperature.